This invention relates generally to the field of magnetic data storage devices, and more particularly, but not by way of limitation, to measuring track mis-registration for a disc drive.
Disc drives are used for data storage in modern electronic products ranging from digital cameras to computer systems and networks. A typical disc drive includes a head-disc assembly (HDA) housing the mechanical portion of the drive, and a printed circuit board assembly (PCBA), attached to the head-disc assembly. The printed circuit board assembly controls operations of the head-disc assembly and provides a communication link between the head-disc assembly and a host device served by the disc drive.
Typically, the head-disc assembly has a disc with a recording surface rotated at a constant speed by a spindle motor assembly and an actuator assembly positionably controlled by a closed loop servo system. The actuator assembly supports a read/write head that writes data to and reads data from the recording surface. Disc drives using magneto resistive read/write heads typically use an inductive element, or writer, to write data to information tracks of the recording surface and a magneto resistive element, or reader, to read data from the information tracks during drive operations.
One type of data recorded to and read from the information tracks is servo data. Servo data, including a physical track identification portion (also referred to as a servo track number or physical track number), written to the recording surface define each specific physical track of a number of physical tracks written on the recording surface. A servo track writer is traditionally used in writing a predetermined number of servo tracks to each recording surface during the manufacturing process. The servo tracks are used by the closed loop servo system for controlling the position of the read/write head relative to the recording surface during disc drive operations.
High performance disc drives achieve areal bit densities in the range of several gigabits per square centimeter (Gbits/cm2). Higher recording densities can be achieved by increasing the number of bits per centimeter stored along each information track, and/or by increasing the number of tracks per centimeter written across each recording surface. Capacity increases gained through increasing the bits per centimeter stored on each track generally require improvements in the read/write channel electronics to enable data to be written to and subsequently read from the recording surface at a correspondingly higher frequency. Capacity increases gained by increasing the number of tracks per centimeter on each recording surface generally require improvements in servo control systems, which enable the read/write heads to be more precisely positioned relative to the information tracks.
Concentric servo tracks written across the recording surface of the disc is the intended result of a servo write process. Each of the concentric servo tracks should be a closed circle with its center coincident with the axis of rotation of the spindle motor. The concentric servo tracks should exhibit consistent track-to-track spacing relative to each other across the surface of the disc. However, in practice, a number of factors disrupt the formation of consistent track-to-track spacing of the servo tracks during the servo write process. The resulting inconsistent track-to-track spacing is referred to as xe2x80x9cwritten-in track squeezexe2x80x9d. Written-in track squeeze is an effect caused by a relative shift in position between the read/write head and the recording surface during the servo write process at a time in the process that the servo data is being written to the recording surface. Written-in track squeeze is an important factor contributing to track mis-registration.
The amount of mis-registration of one track has a direct bearing on the ability of the read element to read data stored on an adjacent track. If the mis-registration of the first track encroaches on the adjacent track by a sufficient amount, erasure of data previously written to the adjacent track can occur when writing data to the first track. An encroachment on data previously written to a track occurring during a write operation to an adjacent track is referred to as xe2x80x9ctrack squeeze.xe2x80x9d The encroachment typically takes the form of a partial overwrite of the data previously written to the track.
Ordinarily, disc drives are designed to withstand a given level of track squeeze. Disc drives that encounter track squeeze beyond design limits cease to operate properly. In establishing a track squeeze standard for the disc drive an additional margin of safety is added to the level of track squeeze the disc drive is designed to withstand. The level of track squeeze represented by the inclusion of the additional margin of safety is established as the track squeeze specification for the disc drive. During the manufacturing process disc drives that fail to meet the track squeeze specification are subjected to costly rework operations.
Since an amount of track squeeze present between adjacent tracks substantially represents the collective effect of a number of factors, during the manufacturing process each factor is typically measured against an empirically determined level of acceptability. The predominant factors causing track squeeze include: the width of the written track; the repeatable runout present in the head disc assembly; the occurrence of non-repeatable runout during a write operation; and the written-in track squeeze. In establishing the empirically determined levels of acceptability for each factor, a level of contribution to track squeeze generally caused by each factor is resolved. Based on the contribution generally caused by each factor, a threshold limit is typically established for each factor. Each threshold is ordinarily combined with an appropriate level of margin to form a level of acceptability for each factor. As each level of acceptability is essentially an acceptance level, disc drives that fail to meet the acceptance level for any of the factors are failed and undergo re-work operations.
However, because track mis-registration is a result of a complex combination of the aforementioned factors, a deficiency in one factor can be compensated by an improved robustness of one or more of the other factors. For example, a disc drive may exhibit a track width greater than the acceptance level width, while at the same time having minimal written-in track squeeze, a small amount of repeatable runout and an acceptable margin for non-repeatable runout. Currently, even though the collective effect of the factors may result in a track squeeze condition that is within the disc drive track squeeze specification, disc drives are failed and subjected to costly re-work because one of the individual factors, taken alone, fails to meet its individual level of acceptability.
Therefore, challenges remain and needs persist for means of measuring the collective result of the combination of factors contributing to track mis-registration for use in determining disc drives having a sufficient level of track squeeze margin for proper functionality, even though one or more of the contributing factors fall outside an empirically determined level of acceptability. It is to this and other features and advantages set forth herein that embodiments of the present invention are directed.
As exemplified by preferred embodiments, the present invention provides for measuring track mis-registration operating performance of a data storage device with a plurality of information tracks for use in predicting proper operation of the data storage device. Track mis-registration is measured by performing DC erasures on three adjacent information tracks selected from the plurality of information tracks. Next, a predetermined bit pattern is written on a middle track of the three adjacent information tracks, which is read to provide a first signal to a variable gain amplifier to generate a first value of the variable gain amplifier. Then, DC erasures are again performed on the information tracks adjacent the middle track, and the predetermined bit pattern is reread from the middle track to provide a second signal to the variable gain amplifier to generate a second value of the variable gain amplifier. The first and second values of the variable gain amplifier are compared to provide a track mis-registration value for use with a predetermined threshold value to determine the functional capability of the data storage device.
These and various other features and advantages, which characterize the present invention, will be apparent from a reading of the following detailed description and a review of the associated drawings.